In a process of manufacturing a semiconductor device such as a Large Scale Integration (LSI) circuit, a fine circuit pattern is formed on a surface of a semiconductor wafer. A lithography technology is used to form the circuit pattern. In the lithography technology, a photomask (also called a reticle) that is an original plate of a circuit pattern is irradiated with exposure light, whereby the circuit pattern is transferred onto the surface of the semiconductor wafer through a reduction optical system.
In recent years, with increased integration and speed of LSI circuits, further reduction in the size of the circuit pattern is rapidly being progressing. To increase the resolution of lithography, Japanese Laid-Open Patent Publication No. 2009-212220 describes extreme ultraviolet (EUV) lithography in which EUV light having a wavelength of approximately 3 nm to 30 nm is used as a light source. In the EUV lithography, a reflective mask is used because a significant decrease in intensity of the EUV light occurs when using a transmissive mask such as a glass reticle.
A Mask blank that is used to form a reflective mask includes a multilayer reflective film and an absorption layer that are stacked on a glass substrate, such as quartz glass or low thermal expansion glass substrate. The multilayer reflective film has a high reflectance to the wavelength of an exposure light source. The absorption layer absorbs the EUV light used as exposure light. The absorption layer of the mask blank is partially removed through electron beam lithography and etching to form an absorption pattern. The absorption pattern and the multilayer reflective film form a desired circuit pattern. The EUV light is absorbed by the absorption pattern and is reflected by the multilayer reflective film in a part in which the absorption layer is removed. As a result, an optical image reflected by the reflective mask is transferred on a semiconductor wafer after passing through a reflection optical system.
In lithography, the finer the circuit pattern, the lower the uniformity of dimensions. One of factors of decreasing the uniformity of dimensions is a focus error (variation in focus) in an exposure apparatus. Such a focus error causes a pattern formed to have a variation in dimensions. In particular, in an isolated pattern (non-dense pattern) having a narrow focus margin and a small depth of focus (DOF), a variation in dimensions caused by a variation in focus greatly appears. This may cause a significant reduction in width dimension. Further, in the EUV lithography, when the size of the isolated pattern is reduced, a variation in dimensions significantly appears.